1. Field of the Invention
The invention relates generally to the field of electron beam welding of dissimilar metals. More specifically, the invention relates to a method of electron beam welding of a cuprous metal to a ferrous metal. More particularly, the invention relates to welding copper to stainless steel.
2. Discussion of the Related Art
In an electron beam welding process a concentrated stream of high-energy electrons is directed to the abutting surfaces or interface of the work pieces to be welded. This high-energy electron bombardment causes rapid heating, forming a vapor hole surrounded by molten metal. The work piece is then moved away from the beam. The molten metal flows away from the hole and solidifies to form the weld.
This technique is highly satisfactory for welding relatively thin pieces of metal together. The process is also used to weld large structural members.
In general, an electron beam welding apparatus is provided with an electron gun and a driving table disposed in a vacuum chamber. The electron gun emits an electron beam which is directed on an interface between the two work pieces that melts and welds the metals at the abutment. Shortcomings have been found in the application of electron beam welding to dissimilar metals. Dissimilarity of metals can cause beam deflection resulting in unwelded spots, voids, cracks and other defects in the weld.
Difference in physical properties often hampers the welding of two dissimilar metals. Different melting points, different thermal conductivities, different thermal expansion coefficients, and different crystal structures can each cause problems. For example, when welding copper to stainless steel, excessive melting of copper and intermixing with steel in the weld zone can cause cracking, voids, and weld root porosity. These defects are detrimental to the integrity of the weld. Consequently, copper is not routinely joined to stainless steel by welding.
Explosion bonding is a method used for joining dissimilar metals. In explosion bonding the two metals are exposed to a series of shock waves that deform the abutting surfaces, physically interlocking the two metals. However, explosion bonding requires expensive, specialized equipment and inconvenient, controlled conditions.
It would be desirable to use electron beam welding (EBW) for joining dissimilar metals because the equipment is commonly available and the method is cost effective. In high-voltage, high-vacuum electron beam welding, the heat input to the weld interface can be precisely adjusted to allow control of the amount of metal intermixing. However, even with controlled heat input, the production of a mechanically sound joint between two dissimilar metals is difficult. Prior art electron beam welding methods have demonstrated limited success in applications where both mechanical integrity and electrical conductivity of the resulting work piece are required.
Inventor was motivated to find a method of welding dissimilar metals that overcomes the disadvantages of the prior art. Inventor searched for an electron beam welding method of joining copper and stainless steel to produce a high-quality, mechanically sound joint having high electrical conductivity.
The invention is a method of welding a copper work piece and a stainless steel work piece by an electron beam. According to the invention a copper work piece and a stainless steel work piece are placed in contact to form an interface therebetween. An electron beam is directed on the stainless steel work piece at a first point located a first distance from the interface. The electron beam is then moved on the work pieces in a direction that traverses the interface to a second point on the copper work piece a second distance from the interface. Then, the electron beam is moved on the copper work piece in a direction essentially parallel to the interface for a third distance to a third point, while maintaining the electron beam at the second distance from the interface. The electron beam is then moved on the work pieces in a direction that traverses the interface from the copper work piece to the stainless steel work piece to a fourth point. The electron beam is then terminated. A weld is thereby produced at the interface.